The present invention is broadly concerned with NOx abatement, particularly with NOx abatement in the exhaust gas of lean-burn internal combustion engines.
It is well known in the art to utilize catalyst compositions, such as those commonly referred to as three-way conversion catalysts (xe2x80x9cTWCxe2x80x9d) to treat the exhaust gases of internal combustion engines. Such catalysts have been found to successfully promote both the oxidation of unburned hydrocarbons (xe2x80x9cHCxe2x80x9d) and carbon monoxide (xe2x80x9cCOxe2x80x9d), and the reduction of nitrogen oxides (xe2x80x9cNOxxe2x80x9d) in the exhaust gas, provided that the engine is operated at or close to an air/fuel ratio balanced for stoichiometric combustion (i.e., between about 14.4 and about 14.7; xe2x80x9ccombustion stoichiometryxe2x80x9d). Currently, much research is underway to develop gasoline fueled engines, such as those used for passenger automobiles and the like, to operate under lean conditions, for at least part of the time, to achieve better fuel economy. Under lean conditions, the air to fuel ratio (A/F) and combustion mixture supplied to the engine are maintained above combustion stoichiometry so that the resulting exhaust gases are xe2x80x9cleanxe2x80x9d, i.e. the exhaust gases are relatively high in oxygen content and relatively low in reductant content, e.g. HC, CO and/or hydrogen.
Although lean burn engines provide enhanced fuel economy, they have the disadvantage that a conventional three-way conversion catalyst cannot adequately abate the NOx component of pollutants in the gas stream, as is required in order to meet increasingly stringent emission control regulations being implemented throughout the world. Efforts to increase NOx removal from exhaust gas emissions have resulted in the development of NOx adsorbers which typically comprise a sorbent material to remove NOx from a fuel-lean gas stream. While NOx adsorbers have been shown to remove NOx pollutants from exhaust gases, they suffer from the serious drawback that they are readily poisoned by sulfur also present in exhaust gases.
Without being bound to any particular theory of operation, it is believed that, under lean conditions, NOx present in the gas stream is sorbed onto the NOx adsorber in the form of stable metal nitrates, thereby removing NOx from the gas stream to be discharged to the atmosphere. A catalytic component is believed to promote such sorption. During rich conditions (i.e., high reductant content) reductants in the gas stream decompose the stable metal nitrates in the NOx adsorber, releasing NOx from the adsorber. The released NOx contacts a NOx abatement catalyst in the presence of reductants and, under suitable conditions (e.g., sufficiently elevated temperature), the NOx is reduced to nitrogen or to nitrogen and N2O.
By a mechanism similar to that for the formation of the stable metal nitrates, sulfur oxides present in the gas stream are sorbed onto the NOx adsorber in the form of very stable metal sulfate salts. Unlike the storage nitrates which readily decompose under rich conditions, decomposition of the metal sulfates requires a combination of rich gas and elevated operating temperature. This combination of conditions may not occur with sufficient frequency under normal operating conditions for typical lean bum gasoline and diesel engines. As a result, the NOx storage sites are gradually poisoned by sulfur, and the NOx conversion capability of the catalyst gradually degrades to unacceptable levels for efficient exhaust purification.
The NOx adsorber compositions disclosed herein provide enhanced recovery from poisoning by sulfur compounds, while retaining their NOx abatement efficiency.